الفهرس 
Introduction
Effect of genotype and season on egg production of naked neck chickens under subtropical climatic conditionsOnly 14 pages are availabe for public view
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Abstract
Four experiments were carried out at the Poultry Research Farm, Animal and Poultry Production Department, Faculty of Agriculture Assiut University.
EXPERIMENT I
The main purpose of this experiment was to evaluate the productive performance of naked neck chickens (NaNa and Nana) under the prevailing conditions in Assiut governorate (subtropical conditions). A total number of 775 laying hens represent three genotypes; homozygous (NaNa), heterozygous (Nana) naked neck and normally feathered (nana) were used in this study. During the experimental period (13 laying periods/ 28 days each), egg production (egg number, egg weight and egg mass), egg quality and some physiological parameters were measured and recorded throughout the experiment.
The obtained results were summarized as follow:
Body weight:
1- Body weight of the naked neck birds (Na/-) was greater than the normally feathered (nana) birds at the age of sexual maturity. Differences between genotypes were highly significant (p<0.01).
2- At 40 and 52 weeks of age, body weights for naked neck (Na/-) birds were heavier than normally feathered (nana) birds, however, the differences between the genotypes were insignificant. At 72 weeks of age, body weight of naked neck birds (NaNa and Nana) were heavier than the normally feathered and the differences among genotypes were significant (p<0.05).
It could be concluded that the presence of Na gene significantly increased body weight of naked neck birds compared to normally feathered type.
Egg production parameters:
Age at sexual maturity:
1- The normal feathering (nana) genotype reached sexual maturity earlier than NaNa and Nana sibs. The mean values were 154.15, 154.56 and 151.57 g for NaNa, Nana and nana genotypes, respectively. There was a significant (P<0.05) difference between naked neck and normally feathered genotypes for age at sexual maturity.
1- Laying rate (%):
1- The maximum laying rates were observed from the 2nd to the 4th laying periods, whereas the lowest rates were recorded mainly at the 12th and 13th laying periods.
2- The results indicated that naked neck (Na/-) genotypes had higher laying rates than normal feather genotype (nana).
3- The naked neck genotypes (Na/-) showed more persistency than normal birds especially at the last two laying periods. The averages of laying rates were about 58, 55 and 54 % for the NaNa, Nana and nana, respectively.
2- Egg number:
1- Regarding to egg number, the results showed superiority of the naked neck chicken due to the presence of Na gene. The maximum egg numbers were recorded from the 2nd to the 4th laying periods, whereas the lowest rates were observed mainly at the 12th and 13th laying periods. Similar effect was observed for the total egg number (TEN) where, the NaNa and Nana laid more eggs than their normal birds (na/na) by about 7.5% and 1.1%, respectively.
3- Egg weight:
1- The naked neck birds (Na/-) had heavier egg weight than normal feather (nana) counterparts at different ages. The maximum egg weights were recorded from the 4th to the 13th laying periods, whereas the lowest rates were observed during the first 3 laying periods. The naked neck (Na/-) had significantly heavier initial egg weight when compared with normal hens (nana).
4- Egg mass:
1- The presence of Na gene improved total egg mass (TEM) for NaNa and Nana genotypes by about 11.81% and 4.09%, respectively.
Effect of genotype on egg production parameters at different seasons:-
Laying rate:
1- The presence of naked neck (Na/-) gene significantly (P<0.01) increased egg laying rate in summer, autumn and spring compared to normal feathering chickens (nana). The averages of laying rates during summer were 60.93, 54.11 and 53.32 % for NaNa, Nana and nana genotypes, respectively.
2- In autumn, the mean values were 61.50, 52.11 and 58.12 % for NaNa, Nana and nana genotypes, respectively. In spring the mean values were 55.33, 50.63 and 50.14 for NaNa, Nana and nana genotypes, respectively. However, in winter there was no significant difference between genotypes for egg laying rate. The average of laying rate (ALR) for the NaNa, Nana and nana was about 58, 55 and 54 %, respectively.
Egg number:-
1- Birds carrying the naked neck (Na/-), gene produced higher egg number compared to normal feathering (nana) counterparts in different seasons except in winter, where the differences between genotypes were insignificant. Similar effect was observed for total egg number (TEN) where, the Na/Na and Na/na laid more eggs when compared with normal birds (na/na) by about 7.8% and 1.0%, respectively.
Egg weight:-
1- Birds carrying the naked neck (Na/-) genes had significantly heavier egg weight (P<0.01) compared to normal feathering (nana) birds. However, there was no significant difference between genotypes for egg weight in spring.
Egg mass:
1- The presence of (Na/-) gene significantly (P<0.01) increased egg mass compared to nana genotype in summer, autumn and spring; however, differences in winter were not significant.
Egg components and shell quality traits.
Yolk percentage:
1 -The effect of genotype on yolk percentages was insignificant. The averages of yolk percentage by genotype were 31.79, 32.06, and 31.65% for the NaNa, Nana, and nana, respectively.
2- The effect of age on the percentage of egg yolk was highly significant. The percentage of egg yolk at 40, 52 and 72 wks of age were 31.27, 32.95 and 31.27%, respectively. In addition, the yolk percentage increased with the advanced of age till 52 wks, thereafter, the yolk percentages declined up to 72 wks of age which reflect the end of egg laying season. Interactions between genotypes and ages considering yolk percentage were not significant.
Albumen percentage:
1- The averages of albumen percentage for genotypes were 58.33, 58.20, and 58.57 % for NaNa, Nana, and nana, respectively. The effect of age on albumen percentage was highly significant (P<0.01). The results showed that the albumen percentages at 40, 52 and 72 wks of age were 58.51, 57.04 and 59.55 %, respectively.
2- The lowest percentage of albumen at 52 wks of age coincides with the high percentage of yolk at this age. Furthermore, increased hen age from 40 to 52 wks old decreased albumen percentage and increased yolk percentages. The interaction between genotype and age was not significant.
Shell percentage:
1- No significant effect of the genotype on egg shell percentage was found. Normal feathering (nana) birds recorded the highest shell percentage value 9.9% as compared to NaNa and Nana genotype 9.8 and 9.7%, respectively. The reduction in shell percent for NaNa and Nana led to significant reduction (P<0.05) in egg breaking strength and shell thickness.
2- The effect of age on shell percentage was highly significant (P<0.01). It was found that shell percentages were 10.22, 10.02 and 9.20% at 40, 52 and 72 wks of age, respectively. Shell percentage decreased with advancing age as result of decrease calcium absorption and its metabolism. The effect of interaction between genotype and age on shell percentage was not significant.
Shell quality traits:
Shell thickness.
1- The effect of genotypes on shell thickness was insignificant. The shell thickness was 0.38, 0.38 and 0.39 mm for NaNa, Nana and nana genotypes, respectively.
2-The effect of age on shell thickness was highly significant and means were 0.39, 0.39 and 0.37 mm at 40, 52 and 72 wks of age, respectively. The lowest egg shell thickness was observed at the end of egg laying season (72 wks of age). The decrease in both shell percentage and thickness reflect aging characteristics.
3- The effect of interaction between genotype and age on shell thickness was not significant.
Shell strength.
1-The presence of Na gene significantly decreased shell strength (kg/cm2) of NaNa and Nana genotypes. The averages of shell strength were 3.71, 4.24, and 4.31 kg/cm2 for NaNa, Nana and nana genotypes, respectively.
2- Averages of shell strength at different ages were 4.48, 4.16 and 3.62 kg/cm2 at 40, 52 and 72 wks of age, respectively. It is observed that the highest shell strength was at 40 wks of age and the lowest was at 72 wks of age. Shell strength in Sharkasi chicken showed a linear manner with advancing age till the end of egg laying season.
Anatomical and physiological parameters:
Anatomical parameters:
1- No significant interaction was observed between the main factors (genotype and age) in the studied traits (Carcass, dressing, giblets, ovary, oviduct percentages and mature ova number).
2- Genotypes and ages showed no significant effect on carcass and dressing percentages. On the other hand, genotypes and age increased the percentages of ovary, oviduct and the mature follicles number.
3- Insignificant increase in reproductive organs coincided with increasing egg number and egg weight in the naked birds compared to those of normal feathering birds.
4- Giblets percent in the naked neck birds (NaNa 5.0% and Nana 4.87 %) showed highly significant increase compared to those of normal feathering (nana 4.7%). The effect of age on giblets percentage was highly significant (P<0.01). It was found that giblets percentages were 4.7, 5.2 and 4.6% at 40, 52 and 72 wks of age, respectively.
Blood constituents:
Heamoglobin concentration:
1- Heamoglobin concentration of NaNa (12.08 mg/100ml) was higher than the other genotypes (Nana 9.68 and nana 8.78 mg/100ml) and there were highly significant differences (P<0.01) among genotypes.
2- The effect of age on heamoglobin concentration was highly significant (P<0.01). The results showed that the heamoglobin concentration at 40, 52 and 72 wks of age were 9.55, 9.87, and 11.12 mg/100ml, respectively. The interaction between genotype and age was significant (P<0.05).
Phosphorus concentration:
1- The differences were insignificant in phosphorus level due to genotype.
2- Plasma phosphorus concentrations at 40, 52 and 72 wks of age were 7.05, 6.42 and 5.53 mg/100 ml, respectively. The differences were highly significant (P<0.01). In addition, phosphorus level decreased with the advanced of age till 72 wks which coincided the end of egg laying season.
3- Interactions between genotype and age was not significant.
Calcium concentration:
1- There was a significant reduction (P<0.01) in plasma calcium level due to the presence of Na gene. Plasma calcium was 14.21, 13.33 and 14.54 mg/100ml for the NaNa, Nana and nana genotypes, respectively.
2- The effect of age on the calcium concentration was highly significant (P<0.01). Calcium concentration at 40, 52 and 72 wks of age were 12.99, 13.88 and 15.21 mg/100ml, respectively. In addition, the calcium concentration decreased with the advancing of age.
3- Interaction between genotype and age considering plasma calcium was not significant.
In general, the achieved results clearly indicate that Sharkasi chickens had favorable effects on egg production performance under subtropical conditions compared to their normal feathering sibs. Therefore, Sharkasi chickens as naked neck birds are preferred for any poultry breeding program in tropical and subtropical regions